Field of the Invention
The present invention relates in general to an apparatus for the generation of electro-mechanical work. In particular although not exclusively the present invention relates to high speed electromagnetic turbines.
Discussion of the Background Art
One of the fundamental principles of physics is the relationship between electricity and magnetism. This relationship was first observed in mid 1800s when it was noted that current passing through a simple bar conductor, placed in an external magnetic field perpendicular to the current flow induces torque. This as each of the moving charges, which comprises the current, experiences a force as a result of the induced magnetic field. The force exerted on each of the moving charges generates torque on the conductor proportional to the magnetic field.
The above discussed basic interactions between electric and magnetic fields are the basic scientific principles which underpin electric motors and generators. One of the simplest forms of electric generator was first exemplified by Michael Faraday, with his use of a device now know as the Faraday disc. Faraday's device consisted of a copper disk rotated between the poles of a permanent magnet. This generates a current proportional to the rate of rotation and strength of the magnetic field. The Faraday disc was in essence the first homopolar generator. Faraday's generator however was exceedingly inefficient due to counter flows of current which limited the power output to the pickup wires, and the effects of parasitic heating on the copper disc.
Despite various advances in design and materials since Faraday's original demonstration, homopolar generators have generally long been regarded as being extremely inefficient. Nonetheless homopolar generators have some unique physical properties that make them desirable for certain applications. Firstly homopolar generators are the only generators that produce a true DC output. Most multi-pole generators are required to commutate or selectively switch into AC windings to get a DC output. In addition to this homopolar generators typically produce low voltages and high currents.
Similarly homopolar motors allow high power levels to be achieved from the motor via the application of a comparatively low voltage power supply. It is this fact that has seen much interest in homopolar motors in a number applications, for example electric vehicles. One example of such a motor under development at the University of Texas utilises a four-pass armature and operates at a peak current of 5,000 A from a 48V battery pack. Full power efficiency is currently at 87% with the majority of losses coming from the brushes. In fact one of the major limitations in homopolar motor design is the losses associated through power transfer via conventional brushes. Brush wear is also a factor, particularly in high speed applications where the brushes have a greater frequency of contact with the armature.
Another factor affecting the efficiency of homopolar motors is the production of drag by eddy currents created within the rotors. Eddy currents occur where there is a temporal variation in the magnetic field, a change in the magnetic field through a conductor or change due to the relative motion of a source of magnetic field and a conducting material. Eddy currents become a particular concern in applications where high speed rotors and large magnetic fields are utilised.
Typical homopolar motors require relatively large magnets or plurality of magnets fields to produce the required field, the size and number of magnets again adds to the overall size and weight of the system. Both size and weight of the motor are critical design considerations in applications such as electric propulsion systems.
Given the benefits of homopolar systems (i.e. a system which utilises a single unidirectional field) it would be advantageous to provide a homopolar system which ameliorates at least some of the aforementioned deficiencies of the prior art.